Packet-Scheduling and Shared Channel Transmission
In wireless communication systems employing packet-scheduling, at least part of the air-interface resources are assigned dynamically to different users (mobile stations-MS or user equipments-UE). Those dynamically allocated resources are typically mapped to at least one Physical Uplink or Downlink Shared CHannel (PUSCH or PDSCH). A PUSCH or PDSCH may, for example, have one of the following configurations:                One or multiple codes in a CDMA (Code Division Multiple Access) system are dynamically shared between multiple MS.        One or multiple subcarriers (subbands) in an OFDMA (Orthogonal Frequency Division Multiple Access) system are dynamically shared between multiple MS.        Combinations of the above in an OFCDMA (Orthogonal Frequency Code Division Multiplex Access) or a MC-CDMA (Multi Carrier-Code Division Multiple Access) system are dynamically shared between multiple MS.        
FIG. 1 shows a packet-scheduling system on a shared channel for systems with a single shared data channel. A sub-frame (also referred to as a time slot) reflects the smallest interval at which the scheduler (e.g., the Physical Layer or MAC Layer Scheduler) performs the dynamic resource allocation (DRA). In FIG. 1, a TTI (transmission time interval) equal to one sub-frame is assumed. Generally a TTI may span over multiple sub-frames.
Further, the smallest unit of radio resources (also referred to as a resource block or resource unit), which can be allocated in OFDM systems, is typically defined by one sub-frame in time domain and by one subcarrier/subband in the frequency domain. Similarly, in a CDMA system this smallest unit of radio resources is defined by a sub-frame in the time domain and a code in the code domain.
In OFCDMA or MC-CDMA systems, this smallest unit is defined by one sub-frame in time domain, by one subcarrier/subband in the frequency domain and one code in the code domain. Note that dynamic resource allocation may be performed in time domain and in code/frequency domain.
The main benefits of packet-scheduling are the multi-user diversity gain by time domain scheduling (TDS) and dynamic user rate adaptation.
Assuming that the channel conditions of the users change over time due to fast and slow fading, at a given time instant the scheduler can assign available resources (codes in case of CDMA, subcarriers/subbands in case of OFDMA) to users having good channel conditions in time domain scheduling.
Specifics of DRA and Shared Channel Transmission in OFDMA
Additionally to exploiting multi-user diversity in time domain by Time Domain Scheduling (TDS), in OFDMA multi-user diversity can also be exploited in frequency domain by Frequency Domain Scheduling (FDS). This is because the OFDM signal is in frequency domain constructed out of multiple narrowband subcarriers (typically grouped into subbands), which can be assigned dynamically to different users. By this, the frequency selective channel properties due to multi-path propagation can be exploited to schedule users on frequencies (subcarriers/subbands) on which they have a good channel quality (multi-user diversity in frequency domain).
For practical reasons in an OFDMA system the bandwidth is divided into multiple subbands, which consist out of multiple subcarriers. I.e., the smallest unit on which a user may be allocated would have a bandwidth of one subband and a duration of one slot or one sub-frame (which may correspond to one or multiple OFDM symbols), which is denoted as a resource block (RB). Typically, a subband consists of consecutive subcarriers. However, in some cases it is desired to form a subband out of distributed non-consecutive subcarriers. A scheduler may also allocate a user over multiple consecutive or non-consecutive subbands and/or sub-frames.
For the 3GPP Long Term Evolution (3GPP TR 25.814: “Physical Layer Aspects for Evolved UTRA”, Release 7, v. 7.1.0, October 2006-incorporated herein by reference), a 10 MHz system (normal cyclic prefix) may consist out of 600 subcarriers with a subcarrier spacing of 15 kHz. The 600 subcarriers may then be grouped into 50 subbands (a 12 adjacent subcarriers), each subband occupying a bandwidth of 180 kHz. Assuming, that a slot has a duration of 0.5 ms, a resource block (RB) spans over 180 kHz and 0.5 ms according to this example.
In order to exploit multi-user diversity and to achieve scheduling gain in frequency domain, the data for a given user should be allocated on resource blocks on which the users have a good channel condition. Typically, those resource blocks are close to each other and therefore, this transmission mode is also denoted as localized mode (LM). However, it cannot be generally assumed that the scheduling entity is aware of the prevalent channel conditions. Therefore, it may be necessary to transmit such Channel Quality Indication (CQI) to the scheduling entity, e.g., from a terminal to a base station. Such information may comprise further parameters related to multiple antenna transmission, such as a Precoding Patrix Indicator (PMI) and a Rank Indicator (RI). Such CQI, PMI, RI therefore should represent the conditions that are applicable to the downlink transmission, i.e., from a base station to at least one terminal.
An example for a localized mode channel structure is shown in FIG. 2. In this example neighboring resource blocks are assigned to four mobile stations (MS1 to MS4) in the time domain and frequency domain. Each resource block consists of a portion for carrying Layer 1 and/or Layer 2 control signaling (L1/L2 control signaling) and a portion carrying the user data for the mobile stations.
Alternatively, the users may be allocated in a distributed mode (DM) as shown in FIG. 3. In this configuration, a user (mobile station) is allocated on multiple resource blocks, which are distributed over a range of resource blocks. In distributed mode a number of different implementation options are possible. In the example shown in FIG. 3, a pair of users (MSs 1/2 and MSs 3/4) shares the same resource blocks. Several further possible exemplary implementation options may be found in 3GPP RAN WG#1 Tdoc R1-062089, “Comparison between RB-level and Sub-carrier-level Distributed Transmission for Shared Data Channel in E-UTRA Downlink”, August 2006 (incorporated herein by reference).
It should be noted, that multiplexing of localized mode and distributed mode within a sub-frame is possible, where the amount of resources (RBs) allocated to localized mode and distributed mode may be fixed, semi-static (constant for tens/hundreds of sub-frames) or even dynamic (different from sub-frame to sub-frame).
In localized mode as well as in distributed mode in—a given sub-frame—one or multiple data blocks (which are inter alia referred to as transport-blocks) may be allocated separately to the same user (mobile station) on different resource blocks, which may or may not belong to the same service or Automatic Repeat reQuest (ARQ) process. Logically, this can be understood as allocating different users.
L1/L2 Control Signaling
In order to provide sufficient side information to correctly receive or transmit data in systems employing packet scheduling, so-called L1/L2 control signaling (Physical Downlink Control CHannel-PDCCH) needs to be transmitted. Typical operation mechanisms for downlink and uplink data transmission are discussed below.
Downlink Data Transmission
Along with the downlink packet data transmission, in existing implementations using a shared downlink channel, such as 3GPP-based High Speed Downlink Packet Access (HSDPA), L1/L2 control signaling is typically transmitted on a separate physical (control) channel.
This L1/L2 control signaling typically contains information on the physical resource(s) on which the downlink data is transmitted (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile station (receiver) to identify the resources on which the data is transmitted. Another parameter in the control signaling is the transport format used for the transmission of the downlink data.
Typically, there are several possibilities to indicate the transport format. For example, the transport block (TB) size of the data (payload size, information bits size), the Modulation and Coding Scheme (MCS) level, the Spectral Efficiency, the code rate, etc. may be signaled to indicate the transport format (TF). This information (usually together with the resource allocation) allows the mobile station (receiver) to identify the number of information bits, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.
In addition, in systems employing Hybrid ARQ (HARQ), HARQ information may also form part of the L1/L2 signaling. This HARQ information typically indicates the HARQ process number, which allows the mobile station to identify the Hybrid ARQ process on which the data is mapped, the sequence number or new data indicator, allowing the mobile station to identify if the transmission is a new packet or a retransmitted packet and a redundancy and/or constellation version. The redundancy version and/or constellation version tells the mobile station, which HARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).
A further parameter in the HARQ information is typically the UE Identity (UE ID) for identifying the mobile station to receive the L1/L2 control signaling. In typical implementations this information is used to mask the CRC (cyclic redundancy check) of the L1/L2 control signaling in order to prevent other mobile stations to read this information.
The table below (Table 1) illustrates an example of a L1/L2 control channel signal structure for downlink scheduling as known from 3GPP TR 25.814 (see section 7.1.1.2.3-FFS=for further study):
TABLE 1FieldSizeCommentCat. 1 ID (UE or group specific)[8-9]Indicates the UE (or (Resource group of UEs) indication)for which thedata transmission is intended.Resource assignmentFFSIndicates which (virtual)resource units (and layersin case of multi-layertransmission) the UE(s)shall demodulate.Duration of assignment2-3The duration for whichthe assignment is valid, could also be used to control the TTI or persistent scheduling.Cat. 2 Multi-antenna related FFSContent depends on the(transport informationMIMO/beam-formingformat)schemes selected.Modulation scheme2QPSK, 16 QAM,64 QAM.In case of multi-layertransmission, multipleinstances may be required.Payload size6Interpretation could depend on e.g., modulation schemeand the number of assigned resource units (c.f. HSDPA).In case of multi-layertransmission, multipleinstances may berequired.Cat. 3 If Hybrid ARQ3Indicates the hybrid ARQ(HARQ)asynchronousprocess process the currenthybridnumbertransmission is addressing.ARQ isRedundancy2To support incrementaladoptedversionredundancy.New data1To handle soft bufferindicatorclearing.If Retrans-2Used to derive synchronousmissionredundancy versionhybrid sequence (to supportARQ isnumberincremental adoptedredundancy)and ‘new data indicator’ (tohandle soft buffer clearing).Uplink Data Transmission
Similarly, also for uplink transmissions, L1/L2 signaling is provided on the downlink to the transmitters in order to inform them on the parameters for the uplink transmission. Essentially, the L1/L2 control channel signal is partly similar to the one for downlink transmissions. It typically indicates the physical resource(s) on which the UE should transmit the data (e.g., subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA) and a transport format the mobile station should use for uplink transmission. Further, the L1/L2 control information may also comprise Hybrid ARQ information, indicating the HARQ process number, the sequence number and/or new data indicator, and further the redundancy version and/or constellation version. In addition, there may be a UE Identity (UE ID) comprised in the control signaling.
Variants
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. For example, the HARQ process number may not be needed in case of using no or a synchronous HARQ protocol. Similarly, the redundancy and/or constellation version may not be needed, if, for example, Chase Combining is used (i.e., always the same redundancy and/or constellation version is transmitted) or if the sequence of redundancy and/or constellation versions is predefined.
Another variant may be to additionally include power control information in the control signaling or MIMO (multiple input-multiple output) related control information, such as e.g., pre-coding information. In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.
In case of uplink data transmission, part or all of the information listed above may be signaled on uplink, instead of on the downlink. For example, the base station may only define the physical resource(s) on which a given mobile station shall transmit. Accordingly, the mobile station may select and signal the transport format, modulation scheme and/or HARQ parameters on the uplink. Which parts of the L1/L2 control information is signaled on the uplink and which proportion is signaled on the downlink is typically a design issue and depends on the view how much control should be carried out by the network and how much autonomy should be left to the mobile station.
The table below (Table 2) illustrates an example of a L1/L2 control channel signal structure for uplink scheduling as known from 3GPP TR 25.814 (see section 7.1.1.2.3-FFS=for further study):
TABLE 2FieldSizeCommentRce assignID (US or [8-9]Indicates the US (or group of UEs)group specific)for which the grant is intendedResource FFSIndicates which uplink resources,assignmentlocalized or distributed, the UE isallowed to use for uplink datatransmission.Duration of 2-3The duration for which theassignmentassignment is valid. The use forother purposes, e.g., to controlpersistent scheduling ‘per process’operation, or TTI length, is FFS.TFTransmission FFSThe uplink transmissionparametersparameters (modulation scheme,payload size, MIMO-relatedinformation, etc.) the UE shall use.If the UE is allowed to select (partof) the transport format, this fielddetermines an upper limit of thetransport format the UE mayselect.
Another, more recent suggestion of a L1/L2 control signaling structure for uplink and downlink transmission may be found in 3GPP TSG-RAN WG1 #50 Tdoc. R1-073870, “Notes from offline discussions on PDCCH contents”, August 2007, incorporated herein by reference.
As indicated above, L1/L2 control signaling has been defined for systems that are already deployed to in different countries, such as, for example, 3GPP HSDPA. For details on 3GPP HSDPA it is therefore referred to 3GPP TS 25.308, “High Speed Downlink Packet Access (HSDPA); Overall description; Stage 2”, version 7.4.0, September 2007 and Harri Holma and Antti Toskala, “WCDMA for UMTS, Radio Access For Third Generation Mobile Communications”, Third Edition, John Wiley & Sons, Ltd., 2004, chapters 11.1 to 11.5, for further reading.
As described in section 4.6 of 3GPP TS 25.212, “Multiplexing and Channel Coding (FDD”), version 7.6.0, September 2007, in HSDPA the “Transport Format” (TF) (Transport-block size information (6 bits)), the “Redundancy and Constellation Version” (RV/CV) (2 bits) and the “New Data Indicator” (NDI) (1 bit) are signaled separately by in total 9 bits. It should be noted that the NDI is actually serving as a 1-bit HARQ Sequence Number (SN), i.e., the value is toggled with each new transport-block to be transmitted.
Channel Quality Indication (CQI)
The section 7.2 of 3GPP TS 36.213 “UE procedure for reporting channel quality indication (CQI), precoding matrix indicator (PMI) and rank indication (RI)” Version 8.2.0, March 2008 defines the reporting of Channel Quality Indicators.
The time and frequency resources that can be used by the UE to report CQI, PMI, and RI are controlled by the eNodeB. For spatial multiplexing, as given in 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, the UE shall determine a RI corresponding to the number of useful transmission layers. For transmit diversity as given in the above-mentioned technical specifications, RI is equal to one.
CQI, PMI, and RI reporting is periodic or aperiodic. A UE transmits CQI, PMI, and RI reporting on a Physical Uplink Control CHannel (PUCCH) for sub-frames with no Physical Uplink Shared CHannel (PUSCH) allocation. A UE transmits CQI, PMI, and RI reporting on a PUSCH for those sub-frames with PUSCH allocation for:
a) scheduled PUSCH transmissions with or without an associated scheduling grant; or
b) PUSCH transmissions with no UL-SCH (Uplink Shared Channel).
The CQI transmissions on PUCCH and PUSCH for various scheduling modes are summarized in the following Table 3, which shows the physical channels for aperiodic or periodic CQI reporting:
TABLE 3Scheduling Period CQI Aperiodic CQI Modereporting channelsreporting channelFrequency PUCCHPUSCHnon-selectivePUSCHFrequency PUCCHPUSCHselectivePUSCH
In case both periodic and aperiodic reporting would occur in the same sub-frame, the UE shall only transmit the aperiodic report in that sub-frame.
Aperiodic/Periodic CQI/PMI/RI Reporting Using PUSCH
A UE shall perform aperiodic CQI, PMI and RI reporting using the PUSCH upon receiving an indication sent in the scheduling grant, hereafter also called channel quality indicator trigger signal. The aperiodic CQI report size and message format is given by the RRC (Radio Resource Control protocol). The minimum reporting interval for aperiodic reporting of CQI and PMI and RI is 1 sub-frame. The sub-band size for CQI shall be the same for transmitter-receiver configurations with and without precoding.
A UE is semi-statically configured by higher layers to feed back CQI and PMI and corresponding RI on the same PUSCH using one of the following reporting modes given in Table 4 and described below:
TABLE 4PMI Feedback TypeNo PMISingle PMIMultiple PMIFeedback TypeWidebandMode 1-2(wideband CQI)PUSCH CQIUE SelectedMode 2-0Mode 2-1Mode 2-2(subband CQI)Higher Layer-Mode 3-0Mode 3-1Mode 3-2configured(subband CQI)Channel Quality Indicator (CQI) Definition
The number of entries in the CQI table for a single TX antenna is equal to 16, as given by Table 5 represented below, which shows a 4-bit CQI. A single CQI index corresponds to an index pointing to a value in the CQI table. The CQI index is defined in terms of a channel coding rate value and modulation scheme (QPSK, 16QAM, 64QAM).
Based on an unrestricted observation interval in time and frequency, the UE shall report the highest tabulated CQI index, for which a single PDSCH sub-frame could be received in a 2-slot downlink sub-frame (aligned, reference period ending z slots before the start of the first slot in which the reported CQI index is transmitted) and for which the transport block error probability would not exceed 0.1.
TABLE 5CQI IndexModulationCoding rate × 1024Efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716 QAM3781.4766816 QAM4901.9141916 QAM6162.40631064 QAM4662.73051164 QAM5673.32231264 QAM6663.90231364 QAM7724.52341464 QAM8735.11521564 QAM9485.5547Precoding Matrix Indicator (PMI) Definition
For closed-loop spatial multiplexing transmission, precoding feedback is used for channel dependent codebook based precoding and relies on UEs reporting precoding matrix indicator (PMI). A UE shall report PMI based on the feedback modes described above. Each PMI value corresponds to a codebook index given in Table 6.3.4.2.3-1 or Table 6.3.4.2.3-2 of 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”. For open-loop spatial multiplexing transmission, PMI reporting is not supported.
As has been described above, the Physical Uplink Shared CHannel (PUSCH) may be used to transmit an aperiodic CQI report, which may be triggered by a special bit (CQI trigger) in the Physical Downlink Command Channel (PDCCH) grant. Usually, in case a data buffer at the UE is non-empty, user data and CQI are multiplexed with each other.
The PDCCH contains a field, the Modulation Code Scheme (MCS) level, ranging exemplarily from 0 to 31, as illustrated in Table 6 below, which points to a row in the MCS/Transport Block Set (TBS) table. This example will serve as a basis for the description of the invention in the following. The resulting TBS and code rate can be computed from the entries in the MCS table and the number of granted resource blocks (RB). The modifications used depending on the allocation size, i.e., the number of allocated resource blocks, are omitted for the sake of simplicity.
TABLE 6MCScoding rate ×Indexmodulation1024efficiencyCommentsCode Rate021200.2344from CQI table0.1171875121570.3057Average Efficiency 0.15332031221930.377from CQI table0.18847656322510.4893Average Efficiency 0.24511719423080.6016from CQI table0.30078125523790.7393Average Efficiency 0.37011719624490.877from CQI table0.43847656725261.0264Average Efficiency 0.51367188826021.1758from CQI table0.58789063926791.3262Average Efficiency 0.663085941043401.3262overlap0.332031251143781.4766from CQI table0.369140631244341.69535Average Efficiency 0.423828131344901.9141From CQI table0.478515631445532.1602Average Efficiency 0.540039061546162.4063from CQI table0.60156251646582.5684Average Efficiency 0.642578131764382.5684overlap0.427734381864662.7305from CQI table0.455078131965173.0264Average Efficiency 0.504882812065673.3223from CQI table0.553710942166163.6123Average Efficiency0.60156252266663.9023from CQI table0.650390632367194.21285Average Efficiency 0.702148442467724.5234from CQI table0.753906252568224.8193Average Efficiency 0.802734382668735.1152from CQI table0.852539062769105.33495Average Efficiency 0.888671882869485.5547from CQI table0.9257812529DL: Implicit TBS signaling with QPSK UL: Transmission using RV130DL Implicit TBS signaling with 16 QAM UL: Transmission using RV231DL: Implicit TBS signaling with 64 QAM UL: Transmission using RV3
In the Table 6 represented above, the MCS indexes 0 to 28 require an extra 2 bits for the coding of the Redundancy Version (RV) on the Downlink (DL). For the Uplink (UL), the RV parameter having the value 0 (RV0) is implicitly used.
It is desirable to define a control signalling scheme, which allows to request a terminal to transmit an aperiodic CQI report to a base station, wherein the report only contains CQI information, i.e., without multiplexing the CQI information with Uplink Shared CHannel data, even in case the data buffer at the terminal is non-empty. In this way, the base station would have an improved control on the content and error resilience of the aperiodic CQI report.